(1) Field of the Invention
The present invention is directed to a device for reducing target strength of an object submerged in a fluid. According to the invention, the device produces a region in the fluid exhibiting a high temperature gradient that induces localized bending of sound rays directed at the object away from the object to thereby effectively cloak it from acoustic detection.
(2) Description of the Prior Art
An acoustic cloaking device generally has two main characteristics: (1) it does not generate significant acoustically sensible reflections, and (2) it bends sound rays directed toward the object sufficiently so that the rays avoid the object being cloaked.
An exemplary acoustic cloaking device may be in the form of a solid spherical shell having selected acoustic properties. While it is difficult to design such a device which will avoid reflections, it is possible to tailor the acoustic properties to achieve a measureable bending of incident acoustic rays.
A sphere having a radius r can be acoustically cloaked with a spherical outer shell having a radius a, and a thickness b-a. The shell may be formed of a meta-material having effective densities ρr and ρφ in the respective radial r and azmuthal φ directions as follows:
                                          ρ            ϕ                    =                                    b              -              a                        b                          ;                            (        1        )            
                              ρ          r                =                                            b              -              a                        b                    ⁢                                                    r                2                                                              (                                      r                    -                    a                                    )                                2                                      .                                              (        2        )            The mathematical details are given in the paper: Phys. Rev. Lett. 100, 024301 (2008).
Meta-materials may be realized with voids containing resonant spring-mass systems. At their resonant frequencies, the internal masses do not move in unison with the bulk material, thereby changing the momentum (and thus the effective mass) in the corresponding direction. Such meta-materials are envisioned with resonant inclusions in porous composite materials.
A device employing meta-materials, acting as an acoustic cloak, would likely only work in a narrow frequency band. Such a device would likely be defeated by a broadband waveform. In addition, in an underwater environment, the properties of meta-materials may change or vary with pressure, temperature and the like, thereby possibly reducing performance or rendering the device inoperable.
The speed of sound in a fluid varies with temperature. FIG. 1 illustrates, in graphical form, the speed of sound c (m/s) in seawater at atmospheric pressure and at a salinity S of 35 parts per thousand (PPT) versus temperature T (° C.). At atmospheric pressure and salinity in parts per thousand (PPT), the speed of sound c is governed by the expression:c=1449.2+4.623T−0.0546T2+1.391(S−35)+ . . . ,  (3)where S is the salinity in parts per thousand (PPT).
FIG. 2 illustrates Snell's law for a first incident ray R1 crossing a boundary B between media 2 and 4 each having different acoustic characteristics c1 and c2. Sound propagation can be represented as an incident ray R1 that bends at the boundary B between the media due to changes in speed of sound c1 and c2 in the respective media according to Snell's Law:
                                          sin            ⁢                                                  ⁢                          θ              1                                            c            1                          =                              sin            ⁢                                                  ⁢                          θ              2                                            c            2                                              (        4        )            where θ1 is the angle of incidence of the incident ray R1 at the boundary B, and θ2 is the angle of refraction or the degree to which the refracted ray R2 bends as it crosses the boundary B. A single transition from the first medium 2 to the second medium 4, as shown, leads to a change in the direction of the ray in the form of a discrete angle. A continuously varying sound speed can be broken down into a large number of very thin layers or sub-bands for analysis.
Applying Snell's Law to each, in the limit, sound bends continuously towards the region of slower speed. The curvature k of a ray deflected by the continuously changing (gradient) speed c in a medium, follows the expression:
                                          k            2                    ⁢                      c            2                          =                                                                          ∇                c                                                    2                    -                                                                                                          ⅆ                    c                                                        ⅆ                    s                                                                              2                        .                                              (        5        )            where s is the arc length along the ray in the medium. The expression provides a way to estimate temperature gradients required for an acoustic cloaking device.
Conventional sonar, shown in FIG. 3, uses an acoustic pulse to detect an object 10 having an outer surface 12, located in an underwater environment 14. An incident acoustic ray 16 is directed towards the object from a source and is reflected as ray 18. The reflected ray 18 is then sensed by a detector. An uncloaked or unshielded object is relatively easy to detect by conventional sonar when the object produces strong reflections, i.e., when it has a relatively high target strength. The effectiveness of conventional sonar may be significantly reduced by absorbing or deflecting incident rays, thereby reducing the target strength of the object.